The substantia nigra pars reticulata (SNr) is a key basal ganglia output nucleus. Parkinsonian motor symptoms are often associated with abnormalities in SNr GABA neuron firing intensity and/or pattern. These neurons receive a dense serotonin (5-HT) innervation that may regulate their membrane potential and firing. The receptors and ion channels involved in this important 5-HT regulation are not fully understood. This R03 proposal seeks to answer the following two important questions: (1) does 5-HT2C receptor (5-HT2C-R) mediates the bulk of 5-HT excitation in SNr GABA neurons? (2) Because HT2C-R is coupled to Gq/11 protein, not any ion channel, so what ion channel(s) mediates the effects of HT2C-R-induced excitation in SNr GABA neurons? In other word, what is the effector channel for HT2C-R that changes the membrane potential in these neurons? To answer these questions, we have performed a series of preliminary electrophysiological and molecular studies. Our data show that SNr GABA neurons selectively express TRPC3 channels. These channels are tonically active and mediate a linear Na+dependent inward current with a reversal potential around -35 mV. Activation of HT2C-R also induces a virtually identical linear Na+dependent inward current with a reversal potential around -35 mV. Inhibition of TRPC3 channels blocks HT2C-R-induced excitation. Based on these preliminary data, we propose that HT2C-R mediates the bulk of 5-HT excitation in SNr GABA neurons and TRPC3 channels serve as the effector channel for G protein-coupled 5-HT2C-R. 5-HT2C-R activation by endogenous 5-HT enhances the tonically active TRPC3 channel, induces an inward current and depolarization that facilitates the regular firing pattern and intensity in basal ganglia output neurons that are critical to movement control. Results from the proposed project will advance our understanding of the basal ganglia movement control neuronal circuitry. Equally important, since Parkinson's disease is caused by dopamine neuron degeneration with additional abnormalities in the serotonin system, our experiments will also provide novel insights into the cellular mechanisms of pathoneurophysiology of Parkinson's disease and provide scientific bases for better treatments. Additionally, our novel concept may also have broad implications on how G-protein-coupled neurotransmitter receptors affect the electrical activity in neurons in other brain areas.